The invention relates to a process for the continuous conversion of meta-kaolin into very finely-divided, low-grit, aqueous zeolitic sodium aluminosilicate of the composition: EQU 0.9 to 1.1 Na.sub.2 O:1 Al.sub.2 O.sub.3 :1.8 to 2.3 SiO.sub.2
of which at least 99.8% by weight have a particle size less than 25.mu. and a high cation exchange capacity and which occurs in the form of an aqueous alkaline suspension,
(a) by reacting meta-kaolin with sodium hydroxide at elevated temperatures, PA1 (b) in an aqueous alkaline suspension which has a composition corresponding to molar ratios of: EQU 1.5 to 5 NaO:1 Al.sub.2 O.sub.3 :1.8 to 2.3 SiO.sub.2 :40 to 200 H.sub.2 O PA1 (1) The crystallization of alkali metal aluminosilicate gels, which are formed by reaction of an aqueous alkali metal aluminate solution with an aqueous alkali metal silicate solution in the presence of excess alkali. In this manner cation-exchanging products can be obtained which contain less than 100 ppm of disturbing impurities. These processes, however, are relatively expensive, as the technical products used as aluminate and silicate components must first be prepared from other raw materials. PA1 (2) The conversion of possibly activated mineral aluminosilicate components in highly alkaline solutions. PA1 (a) reacting meta-kaolin with sodium hydroxide at elevated temperatures, PA1 (b) in a first aqueous alkaline suspension having a composition corresponding to the molar ratios of: EQU 1.5 to 5 Na.sub.2 O:1 Al.sub.2 O.sub.3 :1.8 to 2.3 SiO.sub.2 :40 to 200 H.sub.2 O, PA1 (c) recovering said aqueous, alkaline suspension of zeolitic sodium aluminosilicate, the improvement consisting of: PA1 (d) slowly heating said first suspension from a charging temperature of 50.degree. C. or below to a zeolitization temperature in the range of from 70.degree. to 100.degree. C., with a temperature rise averaging 20.degree. C. within a period of from two to ten minutes, PA1 (e) feeding the first suspension continuously into a reactor heated to said zeolitization temperature and having progressively, separately zoned mixing areas with a stage-like effect and having at least seven stages while mixing said suspension sufficiently in the respective states to avoid sedimentation, PA1 (f) passing said heating suspension through the reactor at a preselected temperature in the range of from 70.degree. to 100.degree. C. at such a rate and time until the degree of crystallization of the zeolite sodium aluminosilicate, determined by X-ray, has reached at least 80% of the theoretically possible crystallinity, and PA1 (g) continuously removing an aqueous, alkaline suspension of zeolitic sodium aluminosilicate from the end opposite of the intake end of the reactor.
which is obtained by mixing meta-kaolin with aqueous soda lye.
The so-called zeolites form a mineral class of alkali metal aluminosilicates with water of crystallization whose aluminosilicate lattice has a defined pore and spatial structure. Synthetic zeolites have gained increasing technical importance and are used, for example, as catalyst supports in chemical processes, as drying, separating or adsorption agents for solvents and gases ("molecular sieves") and as heterogeneous inorganic builders in detergents and cleaning agents. Depending on the purpose of use, structurally different zeolite types as well as different degrees of dryness and purity thereof are required. Normally, such zeolites are produced first in their sodium form and, if desired, are then transformed into other forms by cation exchange.
With regard to the above-mentioned applications, in particular zeolitic sodium aluminosilicate of the NaA type has gained especial technical importance. The chemical composition of this zeolite type corresponds approximately to the empirical formula: EQU 0.9 to 1.1 Na.sub.2 O:1 Al.sub.2 O.sub.3 :1.8 to 2.3 SiO.sub.2 :0 to 6 H.sub.2 O
The characteristic X-ray diffraction diagram of zeolite NaA has been described, for example, in U.S. Pat. No. 2,882,243.
For most technical applications, a very finely-divided zeolite with a grain size distribution as narrow as possible with grain sizes below 10.mu. is preferred. In particular, for use of zeolite NaA in detergents and cleansing agents, there is the additional requirement that the portion of particles having a particle size above 25.mu. should not be more than 0.2% by weight, and that its cation exchange capacity should be as high as possible. The particle fraction larger than 25.mu., referred to in the following as "grit," can be determined by wet screening according to Mocker (DIN 53 580). For the use of zeolite NaA in detergents,a grit content of less than 0.1% by weight is often desirable. In this case, the determination of the grit content by a modified method, pressureless wet screening on a 50.mu. screen, may be expedient.
In the production of zeolitic alkali metal aluminosilicates, two synthesis methods different in principle may be followed. These are:
This conversion of solid, preferably mineral aluminosilicates into zeolitic alkali metal aluminosilicates by treatment with alkali will be referred to in the following as zeolitization.
By using mineral aluminosilicates, widely-distributed in the earth's crust, the production of zeolitic alkali metal aluminosilicates can be made considerably cheaper and can be simplified. Suitable for this purpose are in particular minerals of the kaolinite group, such as Kaolinite, Nacrite, Dickite and Halloysite, in the following called "Kaolin." Kaolin is a product of disintegration by weathering of feldspar and is widely distributed in the earth's crust. The composition may vary widely from deposit to deposit. Depending on the completeness of the feldspar weathering and on the geologic history, kaolin contains, besides the main mineral kaolinite, as secondary mineral components, quartz sand, mica, other clay minerals, and in particular non-weathered feldspar. Organic admixtures therewith consist predominantly of bitumins and humins. All these impurities, which are disturbing in many technical uses of kaolin, can be removed to a large extent by elutriation. Such elutriated kaolins, so-called fine kaolins, are available on the market with a kaolinite content of over 90%.
Kaolinite has a theoretical empirical formula: EQU Al.sub.2 O.sub.3 .multidot.2 SiO.sub.2 .multidot.2 H.sub.2 O
However, the chemical composition of commercial kaolin may differ greatly therefrom, in particular because of the above-mentioned mineral impurities. This is noticeable in particular in the molar ratio SiO.sub.2 :Al.sub.2 O.sub.3. Suitable for zeolitization in particular are kaolins with molar ratios SiO.sub.2 :Al.sub.2 O.sub.3 in the range of from 1.8 to 2.3.
The laminated structure of kaolinite, on the one hand, and the basic structure of the zeolites, on the other, are very different from each other. Kaolinite can be zeolitized only after its laminated structure has been destroyed. This transformation of the highly crystalline kaolinite into the X-ray amorphous, so-called meta-kaolinite, also called destructurization, can best be effected by calcining at 550.degree. to 800.degree. C. Too high a calcining temperature leads to "killing" of the kaolin. Mullite phases then develop, which are no longer zeolitizable. In principle, however, the destructurization of the kaolin to meta-kaolin can be effected also mechanically by thorough grinding, or by a wet chemical process, for example, by treating with strong bases. Destructurization by calcining offers the additional advantage, besides a high space/time yield, that during it the above-mentioned organic impurities are burned up.
In this manner, kaolin can be destructurized to meta-kaolin technically both intermittently and continuously. Heretofore, however, the transformation of this meta-kaolin into very finely-divided zeolitic alkali metal aluminosilicates could be effected on an industrial scale only discontinuously by batch processes. The procedure in such a discontinuous zeolitization process has been described, for example, in Donald W. Breck, "Zeolite Molecular Sieves," John Wiley & Sons, New York, 1974, pp. 725-742, particularly pp. 731-738. Additional methods for the production of Zeolite A from kaolin are described, for example, in German published Patent Applications DE-OS 27 15 934, DE-OS 27 22 564, DE-OS 27 25 496, DE-OS 27 43 597, DE-OS 28 23 927, and DE-OS 28 52 674, but these, too, are discontinuous processes.